This invention relates to a suspension unit, and is particularly, although not exclusively, concerned with a suspension unit for use on a tracked vehicle.
A tracked vehicle has a track extending around a series of track guide wheels. At least some of the guide wheels support the weight of the vehicle hull on the section of the track which is in contact with the ground. In this specification, the expression “hull” is used to refer to the main body of the vehicle. The hull serves the same purpose as the chassis of a conventional vehicle, whether or not the chassis is constituted wholly or partially by the bodywork of the vehicle. Consequently, in the context of the present invention, the word “hull” is considered to be equivalent to a vehicle chassis.
The track guide wheels which support the weight of the vehicle on the ground need to be connected to the vehicle hull by a suspension arrangement which enables the track guide wheel to move upwardly and downwardly relatively to the hull.
In a previously considered suspension unit a gas spring is used to support the vehicle load and the ride height of the vehicle is governed by the volume of the gas in the gas spring. While such a suspension unit is suitable for certain applications, when the temperature of the gas changes, the volume of the gas will also change. Such a temperature change will therefore cause the ride height of the vehicle to change. For example, a temperature change of over 40° C. may occur between day and night temperatures in some climates. This may mean that the ride height of a static vehicle may decrease overnight as the temperature drops which can result in the vehicle becoming grounded and therefore immovable.
It is therefore desirable to provide a suspension unit for a vehicle which can maintain a constant ride height over a range of temperatures.